Question

In your book on Talmudic logic, you presented the inference of the common denominator as based on the principle of Ockham’s razor, and that “the intuitive preference is for the simpler possibility, since it is preferable to assume that there is only one factor for a given result, rather than that each of two different factors can independently produce it. This is the accepted explanation for the inference of ‘the common denominator.’”
But in the reference there to your book Two Carts, you explain that there is an a priori principle here (and not merely an estimate, as in Ockham’s razor) that every law must have a single reason, and that “it seems that the assumption we must make at the basis of the determination regarding the illegitimacy of such a refutation is that every law has one and only one factor, and no more. That is, the Oral Torah, which accepts a derivation of ‘what is common to them’ as legitimate, assumes that it is not possible for a law to be caused by two different reasons (which are not specific cases of one general reason), or by two different factors.” 
My question is whether this really is a necessary assumption (and all we have with which to refute is to narrow the scope of the generalization shared by the two or more reasons that produce the result), or perhaps the assumption that if there is an identical result one must make an inference of the common denominator is not necessary either. Beyond that, do you generally think that a cause is always a sufficient and necessary factor, or can it be only sufficient and not necessary?